PHILADELPHIA – The “Straight Talk Express” slammed into an unexpected wall of jeers, boos, hisses and insults yesterday, as protesters repeatedly interrupted a pre-convention speech by U.S. Sen. John McCain.

The catcalling came from a variety of sources, including supporters of Navajo Indians, and comedian Al Franken, who laughed loudly and harshly when McCain praised Texas Gov. George W. Bush.

“If you like, I do not need to continue,” a stunned McCain said at one point, stepping away from the podium ready to quit his speech. But conservative commentator Arianna Huffington urged him to press on.

The hostile reaction was a remarkable contrast for the Vietnam vet, who spent most of the primary season enjoying a hero’s welcome wherever he went.

In a rare departure, McCain bolted from the auditorium minutes after he finished talking – skipping the familiar question-and-answer session and energetic hand-shaking that he thrives on.

McCain, who spoke before an alternative “shadow” convention on the University of Pennsylvania campus, was delivering the first of two big speeches here during the Republican National Convention.

The senator addressed a coalition of campaign-finance advocates, environmentalists and others who say the Democratic and Republican parties have sold their souls to big business.

The platform of the shadow convention calls for an end to the war against drugs, major campaign-finance reform and a re-evaluation of government anti-poverty efforts.

McCain, whose very presence at the shadow convention was a subtle jibe at Bush, has endorsed the governor and plans a three-day surge of events at the end of the week to gin up excitement for his candidacy.

Plus, McCain is clearing a 10-day block of time just before the election.

“Gov. Bush will make an outstanding president,” McCain said during his speech.